What is compost, what is it made of and how is it composed? We reveal everything about the creation, use, and application of compost in the garden.
Humans have been making compost from various types of waste for centuries to fertilize their plants. As early as the 8th century BC, the Greek poet Homer reported that a fragrant heap of dung ripened in Odysseus’ farmyard, which was later spread across the fields. But which special and mysterious superpowers make compost so valuable to humans that it is sometimes referred to as the gardener’s “black gold”? In this article, you will learn more about the formation and origin, properties, and application of compost.
What is compost
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Compost is dead organic material that has gone through what is known as rotting. In the rotting process, decomposition by air-breathing microorganisms takes place first. These break down parts of the organic material in such a way that it escapes into the air in gaseous form as carbon dioxide. The starting materials slowly break down into larger pieces and ultimately even into individual molecules or atoms.
From these “building blocks”, something new is then created in the process of “humification” – that is, the formation of humus, namely the humic acids (or also “humus molecules”). Taken together and chemically combined with clay particles, these form visible crumbs, and flakes that we can recognize as compost or humus. Perhaps you are now wondering what exactly are the differences between humus and compost – a very legitimate question.
In fact, the word “compost” comes from the Latin compositum, which means something like “what has been put together”. The name thus refers to the diverse raw materials that composters – i.e. humans – use for targeted humus formation. So compost is a type of humus. Humus, on the other hand, is also created naturally and is therefore not to be equated with compost.
What is compost made of?
Like everything has organically grown – i.e. animals, plants, fungi, or algae – compostable waste also consists of different proportions of carbon compounds. The cell walls of plants, for example, consist largely of cellulose, hemicellulose, and pectin. All three are carbohydrates, i.e. carbon compounds that also have oxygen and hydrogen attachments and that are linked at the molecular level to form long, stable chains.
If a cell wall is eaten and digested by microorganisms, then actually the same thing happens as when one of our carbohydrates is consumed, for example in the form of a slice of bread: The carbon compounds contained are converted in the cell respiration to generate energy and the end product is carbon as carbon dioxide (CO2) exhaled together with water.
Summary: What is compost?
- Compost is material that has rotted and turned into humus
- The term “compost” refers to the different raw materials of humus put together by humans
- Compost consists of humus molecules that, alone and in combination with clay particles, form visible flakes
- During composting, carbon dioxide is released because microorganisms feed on carbon compounds in the compost
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Properties of compost
In general, it can be said about compost that – depending on the degree of rotting – it is coarse and fibrous or brownish and crumbly and has a pleasant odor that is often perceived as “woody”. It’s about twice as heavy as peat and half as heavy as sand. Compost also has many medium-sized pores, which can improve aeration in the soil as well as the water balance.
Depending on the starting materials, it can be slightly acidic, neutral, or slightly alkaline. The nutrient content depends on the raw materials and the maturation period: it can be very high or very low. Nitrogen, phosphorus, and potassium – the three main nutritional elements of plants – can be in a balanced ratio to one another, but the proportions of phosphorus and potassium are often greatly increased compared to nitrogen. Compost usually contains all the trace elements that plants need.
The effect that compost fertilization has on the soil and the plants can also differ fundamentally: While a ripe, nutrient-poor green compost lastingly improves the soil properties by increasing the humus content, a fresh, nutrient-rich organic compost is better suited for plant fertilization, because it releases many nutrients, but does not lead to an increase in the humus content in the soil. If you are interested in the properties of compost as a fertilizer, you can find out more in this special article.
The degree of rotting of compost
The degree of rotting is the unit of measurement in which the stability of compost against degradation by microorganisms is indicated. So, depending on the degree of rotting, degradation takes place to a different extent in the soil when the compost has been spread. The stability increases with increasing degree of rotting and increasing compost maturity, at the same time the ability to release nutrients decreases.
We find the reason for this in humification: This turns the organic building blocks released after decomposition into new, stable humus molecules that are immune to degradation. Compost raw material has the lowest degree of rotting, namely 1, fresh compost has a degree of rotting of 2 or 3, finished compost has a degree of rotting of 4 or 5.
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Summary properties of compost:
- The properties of compost depend on the raw materials and the degree of rotting
- Is coarse and fibrous, brownish-crumbly, or a mixture of both
- Pleasant, woody smell
- Heavier than peat and lighter than sand
- Has many medium-sized pores that explain the good water and air balance in compost
- The pH is between 6.2 and 8.4
- Nutrient content from very low to 50 percent by volume
- Contains a lot of phosphorus and potassium, usually less nitrogen, as well as trace elements
- Works as a plant fertilizer or soil conditioner – or both
Where does compost come from?
As noted above, compost is a human form of humus. Humus is also created by the processes roughly described above in many natural locations where there is sufficient organic waste and microorganisms. A special rule of thumb applies here: If the living conditions for the microorganisms involved are tolerable but not quite optimal, much more humus is created than nutrients are released.
And here we have an explanation for thick humus layers in forests and bogs: In these locations, microorganisms are usually too acidic or too wet – there are also many other such humus-promoting locations. Humification can also take place in your garden if you are familiar with the control of humus-promoting conditions.
Compost worms and other composters
You have probably already got the right impression that composting is by no means purely mechanical or chemical, but rather a biological process. This is carried out jointly by various members of the soil flora and fauna. Especially in compost, the number of organisms per volume is incredibly high, because here you will find a real feast.
Believe it or not, one cubic meter of compost contains around 10 kilograms of living organisms! Participating groups are bacteria, fungi, and other protozoa, roundworms (nematodes), insects and their larvae, mites, earthworms of various genera, woodlice, millipedes, and snails. Depending on the prevailing living conditions, flora and fauna can be composed differently.
The decomposition, humification, chemical change, mixing, and degradation of organic pollutants are their tasks. Of course, they don’t do this to give us pleasure: rather, they come to feed and reproduce when they find optimal living conditions. And this includes the presence of organic matter, sufficient moisture and oxygen, a weakly acidic or slightly alkaline pH value, and temperatures that are as warm as possible. If all of the organic matter has been converted to humus, they either die or migrate.
Tip – Effective Microorganisms (EM): Various commercially available mixtures of widespread microorganisms (e.g. yeasts and bacteria) are referred to as EM, which is intended to accelerate the necessary metabolic processes when added to compost, in Bokashi fermentation, or sewage treatment plants. For use, the powder mixtures are mixed with a sugar solution and kept warm for a long time.
Then the microorganisms are called “activated” because in the sugar solution they could develop from the initially inactive permanent forms to active microorganisms and multiply strongly. With the sugar water, they are then poured over the material to be processed. Unfortunately, the effect of using EM has not yet been proven, there are still legitimate doubts as to whether the results observed are actually due to the microorganisms or not simply to the sugar solution. For this reason, we cannot give a technically justified recommendation for EM in composting.
Compost from recycling yards
Compost that is sold at recycling centers is recycled from green waste and organic waste from the organic waste bin. Depending on the range, nutrient-poor and nutrient-rich materials are composted and sold together or individually. Green compost is lower in nutrients, while organic compost is more nutrient-rich. There are also various composting systems, some of which process huge amounts of compost.
These range from traditional pile composting through permanently implemented so-called “traveling pile” to pressure-ventilated rotting towers or drums with volumes of up to 1000 cubic meters, which constantly and dynamically circulate the compost. Of course, not every recycling center operates such a system – they are only used in areas where a lot of material needs to be processed quickly.
Tip – hot and cold compost: If a lot of compost material accumulates all at once, as in recycling yards, a new pile, i.e. a compost heap, is set up in one go. As a result, the decomposition processes do not take place in layers (like on a compost heap at home), but rather together with all the material. As a result of the life processes of the microorganisms, a lot of heat is generated. In addition, a large compost pile has its own insulation, which can result in core temperatures of 60 to 80 ° C. Since this phase of intensive rotting lasts for several weeks, the entire volume can be heated up once through systematic repositioning. In this way pathogens and weed seeds are killed, so the compost becomes sterile. A private compost heap only reaches much lower temperatures due to the layer by layer and can therefore unfortunately spread seeds and pathogens of plant diseases.
Make compost yourself
Even without your intervention, humus is constantly being formed in nature. But you can take advantage of the underlying processes and produce a high-quality organic fertilizer or soil improver yourself and also relieve your organic or residual waste bin. In a few keywords, we have summarized the steps in which you can get your own compost. You can also find all the necessary details for correct composting in this special article.
- Choosing a composter: compost rental, quick or thermal composter, roller composter, or worm box.
- Find a suitable location: Partly shaded, protected, in the garden on open and loose, healthy soil.
- Prepare the compost: pile up the compost material; Coarse, firm material, and soft, nutrient-rich material alternate. It may be necessary to sprinkle with lime or nitrogen fertilizer.
- An occasional layer of finished compost or a pot of compost tea has the same accelerating effect as using compost starters or compost accelerators.
- Either layer the compost very carefully or – if necessary and possible – move it once a year to achieve good mixing and aeration.
- Under the best conditions, fresh compost is ready after four to eight weeks, which can be used as plant fertilizer. After at least six months you will get a ready-made compost that has both fertilizing and soil-improving properties. After two to three years you will receive ripe compost, which has a soil-improving effect.
Compost tea is prepared in a similar way to how the effective microorganisms (EM) mentioned above are activated. However, the source of the microorganisms is not a purchased mixture, but a few grams of compost. In water, sugar or syrup are mixed with compost and then left at 25 ° C for about a day. It is important to have a good supply of oxygen, which is ensured by blowing in the air or a stirring system.
The microorganisms contained in the compost are supposed to multiply quickly in the broth. You can then use this to inoculate the soil or compost to promote biological activity. Spraying the plants with compost tea is also recommended in various Internet forums, although the intended benefit for the plants is rarely or only very superficially mentioned. Microorganisms living in the soil find no habitat on the plant and would not colonize them by themselves, which is why a positive effect initially appears unlikely.
The use as a compost starter and to increase microbial activity in planting areas with a simultaneous entry of organic matter (such as mulch) can have a positive effect. This applies in particular to newly set composts in very poor soils with low biological activity. It is questionable whether it would not have at least as great an effect to spread the compost yourself.
Make compost tea yourself:
- Pour 100 liters of rain or well water into a clean container (e.g. a rain barrel). If tap water is used, it must stand for about seven days beforehand; stir occasionally
- Heat the water with an immersion heater (100 to 150 W) to around 25 ° C
- Dissolve 500 g of sugar beet syrup in the water, stir in 250 g of rock flour, stir in 500 g of compost or hang in a large-volume net
- Switch on the pond aerator pump (if possible with a bubbler)
- Leave to stand for 12 to 18 hours, then drain, filter if necessary, dilute with rain, well or stale tap water (in a ratio of 1: 5 for soil treatment, 1: 1 for compost treatment) and apply within four hours
- Thoroughly clean the compost tea bin
Tip: Since many of the described effects of compost tea and EM have not yet been proven beyond doubt, not many commercial companies in agriculture and waste management rely on one of these methods. However, larger-scale products for business are already on the market. So far, however, it has been more of an alternative approach and we would advise against relying on anything its advocate’s tout. In any case, it has been proven over years of private and scientific applications that spreading compost improves the soil.
Summary: where does compost come from?
- Humus arises in various natural locations to varying degrees, it can also find its way into your garden in the form of potting soil – the best example of this is peat
- Compost is always created by the activity of countless, very different microorganisms
- You can buy compost at recycling yards or produce it yourself on your own compost heap
- Effective microorganisms, or compost teas, are brewed to multiply beneficial micro organisms and then diluted to soil and compost. So far, however, the effect has hardly been defined or proven
Spreading compost: how to use it properly
Compost has many beneficial properties that improve soil and plant growth. When applying, however, always bear in mind that compost is a collective term for different mixtures of differently rotted and humidified material and that its properties are accordingly not constant. There are basically the following possible applications:
- Use of ready-made compost to fertilize plants and soil
- Mulching and fertilizing with fresh compost
- Flat spreading of ready-made or ripe compost to improve beds
- Mix with soil or other additives to make your own potting soil
- Lawns and trees benefit if they are provided with compost
Depending on the exact area of application, a different type of compost with different properties is ideal.
Compost in agriculture
Compost can also be used in agriculture. The positive effect on soil health and yields has already been proven in various studies. You should really ask yourself why not much more of the “black gold” ends up on pastures and fields. Like any other fertilizer, it is subject to the Fertilizer Ordinance (DüV, supplement to the Fertilizer Ordinance DüMV), which regulates which quantities may be used at what time on which areas and for the cultivation of which crops.
Farmers, gardeners, and also (fruit) tree nurseries have to keep records of their fertilization. In particular, nitrogen inputs are recorded very precisely and checked on a random basis. Agricultural businesses determine for the determination of the fertilizer requirement:
- The withdrawals from the harvested crop
- The nitrogen content in the soil
- The nitrogen replenishment through soil organic matter
- The nitrogen replenishment from harvest residues from previous crops
- The nitrogen supply from previous green manures
- The nitrogen supply from organic fertilization of the previous three years
This list is compared with the nutrient requirements of the crop and further fertilization is adjusted accordingly. Nitrogen inputs and nitrogen withdrawals are offset against each other annually in the “nutrient comparison” and should balance each other out as much as possible or at least not exceed defined limit values.
Composts are treated as follows in the newly drafted Fertilizer Ordinance of 2017: In the first year after application, 4% of the total nitrogen content is taken into account when determining the fertilizer requirement, in the second and third year only 3% each. This results in the released nutrients that should be available to the cultivated plants. In the nutrient comparison, however, 100% of the nitrogen content is taken into account in the three years.
A sufficient supply of the plants with compost would only be possible if the limit values applicable in the nutrient comparison were exceeded. However, anyone who exceeds these limit values must expect penalties. This inconsistency in the fertilizer ordinance makes the use of compost in agriculture more difficult and is currently still regulated by an individual agreement with the fertilizer authority, but will hopefully soon be resolved.
Summary of compost in agriculture:
- Fertilizing with compost has been shown to have many positive effects on soil health and yield
- Compost is currently very difficult to include in the determination of fertilizer requirements and the comparison of nutrients
- This currently makes the use of compost in agriculture even more difficult
As you can see, there are many benefits to compost. With a few more ingredients, you can mix your own compost yourself. In our special article, you will find instructions for mixing your compost-based potting soil.